1. Field of the Invention
This invention relates to a beam source for forming a thin film on a substrate using molecular beam or jet vapor deposition.
2. Description of Related Art
The formation of a thin film upon a substrate has wide applications in both manufacturing and research in a variety of fields, such as microelectronic devices, photosensitive devices and optical devices. High quality films are formed by highly controlled deposition of materials on to a substrate. Various deposition techniques have been developed for such applications, such as physical and chemical vapor deposition, electron beam evaporation, sputtering, ionized cluster beam deposition, and more recently, ballistic deposition and jet vapor deposition using an inert carrier gas.
Many areas of application, such as integrated circuit fabrication, require deposition techniques that are suitable for deposition of material into high aspect ratio features, and at the same time achieve high productivity and reduced consumable cost per wafer. Such applications further require high uniformity of deposition on the substrate. Thus, these applications require a deposition technique that provides high beam directionality, high deposition rate, and high uniformity. These performance requirements are expected to become more stringent as device packing densities increase.
It is becoming increasingly more difficult to adapt existing vapor deposition techniques to meet these demands for performance. For example, electron beam evaporation techniques generally have a medium deposition rate and uniformity, but relatively poor directionality. Molecular beam techniques, on the other hand, have medium directionality but relatively low deposition rate and uniformity.
To produce a highly directional beam, an unheated collimator is typically used. Such a collimator collimates the beam by permitting vapor molecules traveling in directions within a small solid angle from the collimator axis to pass through, while blocking and trapping molecules traveling in other directions which cause the molecules to strike the collimator wall. This technique can produce a beam of high directionality, but typically results in clogging of the collimator by the deposition material.
Other techniques have been developed that achieve higher deposition rate, directionality and uniformity of deposition. One such technique is ionized cluster beam deposition. However, its implementation has been restricted due to the complexity of the hardware which usually includes an ionizer, acceleration grids and deflection apparatus. More recently, techniques have been developed using a highly directional inert gas stream to carry neutral particles of the deposition material to the substrate. In a ballistic deposition system described in J. S. Reid, R. A. Brain and C. C. Ahn, Ballistic Deposition of Al Clusters Into High Aspect Ratio Trenches, 1995 VMIC 545, neutral clusters 3-5 nm in size are formed in a first chamber by using standard evaporation techniques in an ambient of high purity inert gas maintained at a pressure between 0.5 to several torr. Through a nozzle 1 cm in diameter and 14 cm in length, the clusters and the condensation gas are directed as a highly energetic and directional beam toward the substrate in a second chamber which is evacuated by a Roots pump. Cluster velocities of several hundred meters per second are achieved, corresponding to cluster kinetic energy on the order of 10.sup.4 eV and giving sufficient kinetic energy per atom to produce cluster melting during deposition. Thin film deposition rates of 6 .mu.m per minute can be attained. This process has been used to deposit aluminum into 0.5 .mu.m wide trenches having an aspect ratio of 2:1.
Inert carrier gas has also been used in a jet vapor deposition technique described in B. L. Halpern and J. J. Schmitt, Jet Vapor Deposition, in HANDBOOK OF DEPOSITION TECHNOLOGIES FOR FILMS AND COATINGS 822, R. F. Bunshah, Ed., 1994, and U.S. Pat. No. 4,788,082 to Schimitt, to achieve thin film formation at a rate on the order of microns per minute. This technique involves the use of a sonic jet in a low-vacuum fast flow to transport molecular or cluster-laden vapor to a substrate. The jet source has a nozzle with an exit diameter in the range of several millimeters to 2 cm. Helium or other inert gas is supplied to the nozzle and exits from it as a jet. Typically, the nozzle pressure is several torr, and the downstream pressure is a torr or less. The vapor source is placed in the nozzle near the exit. Compared with other techniques, jet vapor deposition requires a lower degree of vacuum in the substrate chamber where the substrate is located, therefore reducing the costs associated with complicated high vacuum apparatus.
The above references show that highly energetic and highly directional beams of the deposition material may be formed and carried by an inert gas stream to the substrate. Such techniques typically achieve higher deposition rates and beam directionality than traditional molecular beam techniques. However, since a single highly directional jet beam is employed, the substrate typically must be scanned relative to the jet beam source in two directions (generally perpendicular to each other) to form a uniform film, as illustrated in the above reference of Halpern and Schmitt.